1. Field of the Invention
The present invention relates to a process for the production of semiconductor devices. More particularly, the present invention relates to a semiconductor wafer process in which a cleaned substrate with or without one or more component parts formed thereon is preserved in ice made from pure water or purified water after washing with the same water and freezing. The ice-covered substrate can be stably preserved for a long period, and if desired, can be easily and safely conveyed to distant areas or far countries. The present invention can be effectively used in the production of a variety of semiconductor devices such as IC's (integrated circuits), LSI's (large scale integrated circuits), VLSI's (very large scale integrated circuits), and the like. The term "component part" used herein denotes any layer or area in the semiconductor devices, for example, an insulating layer, a protective coating, source and drain areas, a gate electrode area, and other parts found in such devices.
2. Description of the Related Art
In the production of the semiconductor devices described above, one of the most important processes is the cleaning of a semiconductor substrate such as silicon substrate. This is because, any contaminants existing on the cleaned surface of the substrate can adversely affect the characteristics and yields of the resulting devices.
Generally, the cleaning of the semiconductor substrate is carried out in the following basic sequence:
1. Removal of organic substances:
In order to remove organic contaminants such as oil, wax, fingerprints and resist residues, the substrate is treated with organic solvents, surfactants, acids and alkalis. PA1 The substrate is treated with a hydrofluoric acid to remove any remaining oxide coating or natural oxide coating. PA1 The substrate is treated with acids to remove particulate contaminants such as metal, oxides and the like, and ions adhered to the substrate surface. PA1 To remove the chemical reagents used in the preceding cleaning steps, and remove any remaining particles, the substrate is thoroughly washed with pure water. PA1 As the final cleaning step, the water-washed substrate is dried.
2. Removal of oxide coating:
3. Removal of particulates and ionic contaminants:
4. Washing with pure water:
5. Drying:
After the drying step, the substrates are generally placed in a "nitrogen box", which is a preservation container filled with clean nitrogen (N.sub.2) gas. Preservation must be carried out under stable and safe conditions, but the cleaned substrate still suffers from various contamination problems. For example, since the nitrogen box is frequently opened and closed, the cleaned surface of the substrate can become contaminated with dust, oily and fatty droplets, and the like in the atmospheric air. Further, when the substrate is preserved for a long period in the nitrogen box, oxygen and water vapors in the atmosphere are gradually introduced into the box as a result of opening and closing of the box, and can cause the formation of a coating of natural oxide having different thicknesses on the surface of the substrate. These problems must be solved because of their effect on the process conditions, and the resultant reduced yield of the semiconductor devices and variations in the characteristics thereof.
In addition to contamination, other problems arise due to the transportation of the cleaned substrates. Recently, there is a tendency to build factories for producing semiconductor devices in distant areas and different countries, from the aspect of economy, etc. In such cases, since the cleaned substrates cannot be preserved for a long period, for the reasons described above, these factories must be equipped with large scale units for cleaning and washing the substrates. This is very expensive and, therefore, increased the costs of production of the devices. Further, since each factory is equipped with its own cleaning unit, the substrates with the same structure but from different factories, could not cleaned under the same cleaning conditions. The resultant variations in the cleaning of the devices caused variations in the yields and qualities of the semiconductor devices. Furthermore, when the cleaned substrates are transported to distant areas or far countries for further processing, the substrates are sometimes damaged due to frequent vibration, and therefore, particles distributed thereby on a surface of the substrate must be completely removed, for example, by washing with pure water, before further processing. This additional washing step is time-wasting and expensive.
In order to at least partially resolve the above problems, the cleaned substrates were dipped and retained in pure water for a long period. Unfortunately, a deterioration of the water used due to the action of bacteria and microorganisms caused a contamination of the substrate surface.
It is therefore desired to provide an improved process for the production of semiconductor devices such as IC's, LSI's, and VLSI's, in which the drawbacks of the prior art semiconductor water process are eliminated, particularly in the washing step and the subsequent preserving and/or transporting steps. Namely, provision is desired of an improved preservation method for the cleaned semiconductor substrates by which the substrates can be preserved for a long period and/or transported, without contamination of the substrate surface.